This invention relates to the field of extractive metallurgy, and more particularly to an improved hydyrometallurgical process for recovery of precious metals and base metals from ores and other sources.
Conventionally, precious metals such as gold and silver are recovered from ores by leaching with alkaline cyanide solution. By reaction with cyanide ion and oxygen, the precious metal is converted to a cyanide complex (gold cyanide anion) which is taken up in the leaching solution. The precious metal is recovered from the cyanide leachate by any of a number methods, including precipitation with a less noble metal such as zinc, direct electrowinning, ion exchange, carbon adsorption.
While widely practiced on a commercial scale, cyanide leaching suffers from well known disadvantages. Leaching rates with alkaline cyanide solutions are quite slow, contact times in the range of ten to fifteen hours being common in the case of gold ores. Because of the toxicity of cyanide, care must be exercised to maintain cyanide solutions on the alkaline side in order to prevent the release of hydrogen cyanide gas. Severe environmental restrictions must be observed, requiring careful monitoring and control of all process purge streams. Spent cyanide leaching solutions must be subjected to waste treatment operations before discharge to the environment.
In refractory ores, precious metals are contained in a quartz matrix which is difficult to break down for removal of the metal. In many of the other ores which remain available, the precious metal is bound to sulfide minerals and carbonaceous materials, which interfere with leaching by alkaline cyanide or other leaching solutions. Commonly, gold is found locked into refractory minerals such as pyrite or arsenopyrite. Processes have been proposed for oxidation of the latter types of ore prior to recovery of metal therefrom. Environmental restrictions make the use of roasting processes unattractive for this purpose, so that some ore processors have resorted to schemes such as pressure oxidation, in which the ore is contacted with oxygen and sulfuric acid at 160.degree. to 180.degree. C. for 1.5 to 2 hours. In addition to pyrite or arsenopyrite, ores to which pressure oxidation is applicable include stibnite, realgar, orpiment, and berthierite. Details on pressure oxidation and the chemistry of such processes are described by Berezowsky et al., "Pressure Oxidation Pretreatment of Refractory Gold," Minerals and Metallurgical Processing, May, 1984, pp. 1-4. In other processes, chlorine has been used for oxidation of sulfide-containing ores. Biochemical processes have also been developed in which bacteria promote the oxidation of the ore. See Chemical Engineering, June 10, 1985.
Carbonaceous ores typically contain graphitic or activated carbon, and long chain organic compounds similar to humic acids. Adsorption of gold or gold cyanide complexes onto the carbonaceous material interferes with the recovery of gold from carbonaceous ores. Consequently, pressure oxidation techniques have been used to eliminate carbon, typically by oxidation to CO of CO.sub.2, and thereby provide better yields in the extraction of gold from the ore in the form of gold cyanide complex.
A variety of waste treatment processes have been developed for spent cyanide solutions used in the leaching of gold and other precious metals. Some metal processors have employed biochemical treatment using bacteria which are capable of degrading cyanide in mine effluents. Others have developed processes for converting cyanide to relatively nontoxic cyanate. One process uses sulfur dioxide in the presence of a copper catalyst, while various others utilize alkaline chlorination for conversion of cyanide to cyanate. Still another waste treatment process involves contact of the waste solution with hydrogen peroxide. Another method for treatment of cyanide bearing industrial waste effluent involves contact with ozone. See Bremen et al, "oxidation of Cyanide in Industrial Waste-Waters," Enviromental Progress, Vol. 4, No. 1 (February, 1985).
Because of the difficulties in extracting precious metals, especially gold, from refractory and carbonaceous ores, efforts have been devoted to the discovery of improved systems for the leaching of such metals. One process long known to the art is leaching with a bromocyanide solution, which is typically prepared by mixing sulfuric acid, potassium cyanide, potassium bromide, and potassium bromate. This process eliminates the need for oxygen cyanidation. However, while fresh bromocyanide solution may be effective for the treatment of refractory and other ores, bromocyanogen is rapidly decomposed by alkali, so that free alkali must be essentially absent during treatment of the ore. See Hamilton, Manual of Cyanidation, McGraw-Hill, New York (1920). Because of this, and further in view of the fact that potassium bromide is a product of the leaching reaction, the safety and environmental problems associated with cyanide solutions are aggravated by the use of bromocyanide.
More recently, processes have been proposed which use precious metal solubilizers other than cyanide. Thus, for example, thiourea has been proposed as an agent which effects leaching of gold at a rate substantially faster than that obtainable with cyanide. Leaching with thiourea is believed to produce a cationic rather than anionic gold cyanide complex. Because acid systems must be used for thiourea leaching, this process may involve increased equipment costs, at least in some instances. In still other processes, potassium iodide or ammonium polysulfide is used as a lixiviant in place of cyanide.
To accelerate cyanidation, proposals have been made for immersion of a sonic resonance rod in the leaching system, thereby enhancing the rate of diffusion of the leaching solution into the solid ore particles containing the precious metal. Various alternatives to cyanide leaching, certain of the newly developed techniques for enhancing cyanidation, and various of the methods for treating cyanide waste solutions are generally discussed in the aforesaid Chemical Engineering article.
In addition to ores, there is a substantial number of additional sources of precious and other metals which offer the opportunity for economical recovery. In fact, many of these secondary sources are substantially richer than the ores with respect to the content of the metal to be recovered. Gold is available from numerous scrap sources, including wastes from industrial uses, gold plated electronic circuit boards, and as an alloy with copper, zinc, silver, or tin in the karat gold used in jewelry. Silver is available from photographic and X-ray film emulsions, from scrap sterling, and from numerous industrial sources. Platinum, palladium and other platinum metals are available from spent catalysts, as well as other industrial and jewelry scrap sources. There is a substantial need for improved processes for recovery of precious and other metals from all sources, both primary and secondary.
As disclosed in Paterson U.S. Pat. No. 3,412,021, 1-bromo-3-chloro-5,5-dimethylhydantoin is known as an oxidizing biocide for use in water treatment. Patent and other technical literature discloses a number of uses for this and other N-halohydantoin compounds, primarily based on the biocidal properties of these compounds.